1.1 Field of the Invention
The invention relates generally to the field of electromagnetic well logging. More particularly, the invention concerns methods, and devices for their implementation, for reducing and/or correcting for the effects of the borehole on an overall formation resistivity measurement.
1.2 Description of Related Art
Various well logging techniques are known in the field of hydrocarbon exploration and production. These techniques typically employ instruments or xe2x80x9csondesxe2x80x9d equipped with sources adapted to emit energy through a borehole traversing the subsurface formation. The emitted energy interacts with the surrounding formation to produce signals that are detected and measured by one or more sensors on the instrument. By processing the detected signal data, a profile of the formation properties is obtained.
Electromagnetic (EM) induction logging is a well-known technique. Induction logging instruments are disposed within a borehole to measure the electrical conductivity (or its converse, resistivity) of earth formations surrounding the borehole. Conventional induction logging instruments are implemented with antennas that are operable as sources and/or sensors. The antennas are mounted on a support or mandrel and spaced from each other along the axis of the support. These antennas are generally coils of the cylindrical solenoid type and are comprised of one or more turns of insulated conductor wire that is wound around the support. U.S. Pat. Nos. 5,157,605 and 5,905,379 (both assigned to the present assignee), for example, describe induction logging instruments equipped with antennas disposed along a support member. In operation, a transmitter antenna is energized by an alternating current to emit EM energy through the borehole fluid (also referred to herein as mud) and into the formation. The signals detected at a receiver antenna are usually expressed as a complex number (phasor voltage) and reflect interaction with the mud and the formation.
FIG. 1 shows a conventional logging instrument 100 forming part of a well logging system. The instrument 100 is adapted for movement through the borehole and coupled to a surface computer 105 by a wireline cable 110. The formation resistivity profile can be determined in real-time by sending the signal data to the surface as they are acquired, or it can be determined from a recorded-mode by recording the data on a suitable recordable medium (not shown) housed within the instrument 100. As known in the art, the signal data are typically transmitted from the instrument 100 to the surface computer 105 by electronics (not shown) housed in the instrument 100. The signal data may be sent to the surface computer along the wireline cable or by alternate telemetry means. Once received by the surface computer 105, the data can be recorded, processed, or computed as desired by the user to generate a formation profile. The profile can then be recorded on a suitable output record medium. Alternatively, some or all of the processing can be performed downhole and the data can be recorded uphole, downhole, or both.
A coil carrying a current can be represented as a magnetic dipole having a magnetic moment proportional to the current and the area encompassed by the coil. The direction and strength of the magnetic dipole moment can be represented by a vector perpendicular to the area encompassed by the coil. In conventional induction and propagation logging instruments, the transmitter and receiver antennas are mounted with their axes along the longitudinal axis of the instrument. Thus, these tools are implemented with antennas having longitudinal magnetic dipoles (LMD). When such an antenna is placed in a borehole and energized to transmit EM energy, currents flow around the antenna in the borehole and in the surrounding formation. There is no net current flow up or down the borehole.
An emerging technique in the field of well logging is the use of instruments incorporating antennas having tilted or transverse coils, i.e., where the coil""s axis is not parallel to the longitudinal axis of the support. These instruments are thus implemented with antennas having a transverse or tilted magnetic dipole (TMD). The aim of these TMD configurations is to provide three-dimensional formation evaluation, including information about resistivity anisotropy in vertical wells and directional sensitivity that can be used for navigation. Logging instruments equipped with TMDs are described in U.S. Pat. Nos. 6,147,496, 4,319,191, 5,757,191 and 5,508,616.
A particularly troublesome property of the TMD is the extremely large borehole effect that occurs in high conductivity contrast situations, i.e., when the mud in the borehole is more conductive than the formation. When a TMD is placed in the center of a borehole, there is no net current along the borehole axis. When it is eccentered in a direction parallel to the direction of the magnetic moment, the symmetry of the situation insures that there is still no net current along the borehole axis. However, when a TMD is eccentered in a direction perpendicular to the direction of the magnetic moment, axial currents are induced in the borehole. In high contrast situations these currents can flow for a very long distance along the borehole. When these currents pass by TMD receivers, they can cause signals that are many times larger than would appear in a homogeneous formation without a borehole.
U.S. Pat. No. 5,041,975 (assigned to the present assignee) describes a technique for processing signal data from well logging measurements in an effort to correct for the effects of the borehole. U.S. Pat. No. 5,058,077 describes a technique for processing downhole sensor data in an effort to compensate for the effect of eccentric rotation on the sensor while drilling. U.S. Pat. No. 5,781,436 describes a technique for measuring the conductivity of earth formations by making subsurface EM measurements at multiple frequencies and preselected amplitudes. However, none of these patents relates to the properties or effects of TMDs in subsurface measurements.
Thus there remains a need for improved methods and apparatus for reducing or eliminating borehole effects associated with the flow of undesired axial currents along the borehole when using logging instruments implemented with TMDs.
The invention provides an apparatus for measuring a property of an earth formation traversed by a borehole. The apparatus comprises an elongated support having a longitudinal axis; a plurality of antennas disposed on the support such that the magnetic dipole moments of the antennas are tilted or perpendicular with respect to the longitudinal axis of the support. The antennas are adapted to transmit and/or receive electromagnetic energy. The apparatus also includes means for conducting alternating current through at least one of the plurality of antennas to transmit electromagnetic energy; at least one sensor adapted to measure an electrical current value when at least one antenna of the plurality of antennas is transmitting electromagnetic energy; means for calculating a factor from the measured current value; and means for scaling the alternating current by the factor.
The invention provides an apparatus for measuring a property of an earth formation traversed by a borehole, comprising an elongated support having a longitudinal axis; a plurality of antennas disposed on the support such that the magnetic dipole moments of the antennas are tilted or perpendicular with respect to the longitudinal axis of the support, the antennas being adapted to transmit and/or receive electromagnetic energy; means for conducting alternating current through at least one antenna of the plurality of antennas to transmit electromagnetic energy; at least one sensor disposed on the support, each at least one sensor adapted to measure an electrical current value when at least one antenna of the plurality of antennas is transmitting electromagnetic energy; means for calculating a factor from a spacing, along the axis of the support, between a selected one of the plurality of antennas and a predetermined point on the support; and means for scaling the alternating current by the factor.
The invention provides a method for measuring a property of an earth formation traversed by a borehole, comprising passing alternating current through at least one antenna of a plurality of antennas disposed within the borehole to transmit electromagnetic energy, the antennas being disposed such that the magnetic dipole moments of the antennas are tilted or perpendicular with respect to the borehole axis; measuring an electric current value associated with the transmitted energy with at least one sensor disposed within the borehole; calculating a factor from the measured current value; passing alternating current scaled by the factor through at least one antenna of the plurality of antennas to transmit electromagnetic energy; receiving a voltage induced by the transmitted electromagnetic energy with at least one antenna of the plurality of antennas; and calculating the property from the received voltage.
The invention provides a method for measuring a property of an earth formation traversed by a borehole, comprising disposing a plurality of antennas within the borehole such that the magnetic dipole moments of the antennas are tilted or perpendicular with respect to the borehole axis; disposing at least one current sensor within the borehole; calculating factors from spacings, along the direction of the borehole, between selected ones of the plurality of antennas and predetermined points along the borehole; selectively passing alternating currents selectively scaled by the factors through selected ones of the plurality of antennas to transmit electromagnetic energy; calculating at least one factor from a current value measured at the at least one current sensor, the current value being associated with the transmitted electromagnetic energy; passing alternating current scaled by the at least one factor through at least one antenna of the plurality of antennas to transmit electromagnetic energy; receiving voltages induced by the transmitted electromagnetic energy with at least one antenna of the plurality of antennas; and calculating the property from the received voltages.
The invention provides a method for measuring a property of an earth formation traversed by a borehole, comprising disposing a plurality of antennas within the borehole such that the magnetic dipole moments of the antennas are tilted or perpendicular with respect to the borehole axis; disposing at least one current sensor within the borehole; calculating factors from spacings, along the direction of the borehole, between selected ones of the plurality of antennas and predetermined points along the borehole; selectively passing alternating currents selectively scaled by the factors through selected ones of the plurality of antennas to transmit electromagnetic energy; measuring electric current values associated with the transmitted energy at the at least one sensor; and calculating the property from the measured current values.